1. Field of the Invention
Embodiments relate to an organic light emitting display, and an image modification method. More particularly, embodiments relate to an organic light emitting display, and an image modification method that can reduce or eliminate an error of a light emission drive time.
2. Description of the Related Art
A conventional organic light emitting display is a display based upon a principle of selectively emitting light by electrically exciting a fluorescent or phosphorescent compound in organic light emitting diodes (OLEDs). Conventional organic light emitting displays include OLEDs arranged in an n×m matrix.
Each OLED may include a transparent anode, e.g., an indium titanium oxide (ITO) anode, an organic thin-film, and a cathode, e.g., metal. The organic thin-film may be a multi-layer structure, including a light emission layer EML that emits light by coupling an electron with a hole, an electron transport layer ETL that transports an electron, and a hole transport layer HTL that transports a hole. Furthermore, the organic thin-film may include an electron injecting layer EIL that injects a separate electron and a hole injecting layer HIL that injects a hole.
Methods of driving such OLEDs include a passive matrix (PM) method and a metal oxide silicon (MOS) thin-film transistor (TFT) active matrix (AM) method. When using the PM method, lines of anodes and cathodes intersect perpendicularly, and the OLEDS are driven by selecting lines. In contrast, when using the AM method, a TFT and a capacitor are connected to each pixel anode, and the OLEDs are individually driven by maintaining a voltage due to the capacitor capacitance. AM methods include a voltage programming method and a current programming method, depending on the type of signal applied from a data driver.
Depending on image data to be transferred to the OLEDs, driving methods may be divided into an analog drive method and a digital drive method. In the analog drive method, current or voltage corresponding to image data being supplied to an OLED is modulated by a pulse amplitude modulation (PAM) process, with the brightness of the OLED corresponding thereto. In the digital drive method, an amplitude of a current or voltage drive waveform corresponding to image data being supplied to an OLED is modulated by a pulse width modulation (PWM) process, with the brightness of the OLED corresponding thereto.
According to a digital drive method of driving an organic light emitting display, one frame (or field) may be divided into a plurality of sub-frames (or sub-fields). Each sub-frame may include a data write period and a light emission drive period. During the data write period, enable signals may be simultaneously applied to scan lines. During the light emission drive period, an n-bit gray scale may be expressed by drive time of 2n (n=0, 1, 2, . . . , and n−1). For example, if n=4, image data may be between (0000(2)) to 15(1111(2)). Here, image data corresponding to the maximum light emission drive time may be 15(1111(2)), and may generate a maximum pulse width for which the OLED expresses maximum brightness. If image data is 7(0111(2)), a pulse width is half of the maximum light emission control drive time and thus, the brightness is lowered. Thus, 4-bit image data may generate 16 pulse widths, which in turn, express 16 gray scales.
In order to reduce power consumption, the brightness of the entire screen may be reduced using an automatic current limit (ALC), in which an amount of current is controlled when the entire screen is lit with a high brightness by an image signal of one frame. The ALC method determines an average brightness value of an organic light emitting display panel by summing up the total data values for displaying on the organic light emitting display panel. A light emission time may be supplied equally to the organic light emitting display panel during one frame in accordance with an average brightness value. However, because each pixel circuit of the organic light emitting display panel has the same light emission drive time irrespective of a data value of each pixel circuit, an erroneous light emission drive time of each pixel circuit may occur.